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THE EXTERNAL CONDITIONS ON THE WILTING 
COEFFICIENT OF PLANTS 


BY 


CHARLES ORLANDO PEAK 


THESIS 


For the 


DEGREE OF BACHELOR OF ARTS 


PLANT PHYSIOLOGY 


COLLEGE OF LIBERAL ARTS 


UNIVERSITY OF ILLINOIS 


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Table of Contents. 


Page. 
Introduction. --------------- ee 
Watledmn22-—2—2- see ee 7 
Results. ---------+---------- jal 
Conclusions. ---------------- 12 


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I. INTRODUCTION. 


In the complex factors which enter into the make 
up of the environment of the plant the available soil water 
is of great importance. The question has often arisen as to 
just what portion of the soil moisture is available to the plant. 
The quantity available varies with the nature and extent of the 
root system, and the environmental factors surrounding the shoot. 
The water of the soil cannot be removed completely by the plant. 
As the available water is reduced through absorption by the roots, 
the physical attraction of the soil particle for the remaining 
water becomes greater and greater until finally an equilibrium 
is reached between the absorptive powers of the root and the 
attractive force of the soil for the water. When this stage is 
reached wilting because of lack of sufficient water to cover 
that lost by evaporation ensues. 

In the attempt to determine the wilting coefficient, 
described later, of different plants, or of the same plant under 


different conditions, numerous factors of the environment must 


be taken into consideration. Most of the water taken up by the 
plant through its roots is lost by transpiration. Thus we see 
that conditions that increase the rate of transpiration call for 
an increase in the absorption of water from the soil. The effici- 


ency of the roots of different species, or of different individu- 


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als of the same species growing under identical conditions can 

be tested relative to their absorptive powers. The measure of 

the ability to absorb water under trying conditions of soil and 
of atmosphere is the point of wilting of the young shoots. 

The non-available moisture left in the soil at the 
time of wilting of the plant has been termed "the wilting coeffi- 
cient", This method was first used by Sachs (1) in 1859 on 
tobacco plants grown in various soils, and has since been used 
and considerably extended by plant physiologists, physicists, 
and ecologists. The work has been done entirely on plants grow- 
ing in soils of definite moisture content and under normal at-— 
mospheric conditions. No attempt has been made to find the 
effect that a change in the atmospheric conditions - wind, mois- 
ture, etc. - have upon the wilting coefficient. In dealing with 
the question of wilting in plants one is facing the rates or 
proportion between two physiological processes, namely those of 
transpiration and of absorption. These two processes are affected 
a great deal by external conditions. As an example, on a hot 
day a plant will wilt for a time because the absorption of water 
by the roots in insufficient to cover that lost by transpiration. 

In 1912 Briggs and Shantz (2) carried on extensive ex- 
periments with 20 different kinds of soils and one hundred differ- 
ent plants, including hydrophytes and xerophytes. They took a 


long series of readings, but their conclusions are unusual, and 


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warrant further investigation. They concluded that for any 
given texture of soil the wilting coefficient remains a constant, 
regardless of the kind of plant or the conditions under which 
the plant was grown. Instead of considering all the factors 
which cause wilting, they have only considered one factor, and 
that is the moisture in the soil. 

V. H. Blackman (3), working with plants under desert 
conditions reached very different results from those of Briggs 
and Shantz. Blackman found a wide variation under different 
environmental conditions. Some of his readings on plants grown 
under lath shelter, and in the open, varied as much as 40% in 
the moisture content of the soil at the permanent wilting stage. 

A low rate of transpiration for a mesophyte, may be 
very high for a zerophyte. Yet it has been found that mesophytic 
and zerophytic plants transpiring at different rates and grow- 
ing in ae of the same texture, will reduce the soil to the 
same degree of dryness. Blackman gives as a possible reason for 
this that the more actively transpiring plant has a larger ab- 
sorbing surface with which to supply the plant with the necessary 
water. 

In the experiments carried on thus far in regard to the 
wilting coefficient of plants, the investigators have been unable 
to find any marked difference in regard to a difference in the 


absorptive efficiency of roots of different plants. One would 


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expect to find in different plants differences in the power to 
absorb water from the soil and thus reduce soil moisture. Black- 
man (3) says in his article on "The Wilting Coefficient of the 
Soil" - "Whether absorption is considered to be mainly a proto- 
plasmic process or as mainly controlled by the osmotic pressure 
of the cells and the condition of aggregation of the cell colloids, 
there must be marked differences in such factors in different 
plants which one would expect to affect the wilting coefficient." 
That such differences have not been noted is due mainly to the 
conditions under which the experiments have been carried out. 
The soil moisture is reduced to a large extent before it becomes 
the controlling factor in absorption. This would affect the 
plant at a very late and critical stage. 

For a thorough study of the wilting coefficient of 
plants in nature, we must consider along the line of changing 
environment. There may be as many wilting points as there are 
factors in the environment. Caldwell (4) points out that wilt- 
ing, brought about by high rate of transpiration of short duration 
is less serious than wilting brought about by a long period of 
slow transpiration. In the first case the root hairs are only 
plasmolyzed, while in the second they are killed, and the recovery 


of the plant takes place only after new root hairs are formed. 


The relations of the wilting coefficient to the differ- 


ent soils have been brought out very well by the work of V. H. 


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Blackman (3). The coefficient is related directly to the physi- 


cal characteristics of the soil. Thus the smaller the particles 
of soil the lower will be the rate of root absorption. The 
conducting property of the soil must needs play an important 
part, for even in a soil through which the roots are well distrib- 
uted only a small portion of the soil is in actual contact with 
the roots. 

Crump -(5), in his paper on moorland soils, brought out 
the relation of water content to the humus content of the soil. 
He showed that the roots of a plant may occupy several different 
kinds of soil layers, varying in moisture content. Thus he found 
thet the amount of water in the soil was directly related to the 
amount of humus. This water content he called the coefficient of 
humidity. The soil around the roots of a heather moor, though it 
varied largely in water content, its coefficient of humidity varied | 
but very little and could be taken as characteristic of its asso- | 
Ciation. The ground societies of nearly all the plants on the 
moorland could be distinguished by means of this coefficient. \ 
He showed clearly that the wilting coefficient of Eriophorum : 
angustifolium and Collema vulgaris were closely related to the 
humus content of the soil. Thus Crump's coefficient of humidity 
may be taken as the standard index in soils where humus is the 
dominant constituent. 


One of the phases of the problem of wilting coefficient 


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is a study which shall lead to a formula that will apply to 

show the relation between wiltings at various rates of transpira- 
tion on the one hand and the water content and mechanical composi- 
tion of the soil on the other. 

It has been long maintained by plant physiologists, 
ecologists and agriculturists, that plants differ in their power 
to withdraw water from the soil. Until of late no definite ex- 
periments have been carried on where the conditions have been 
definitely under control. Hedgcock (6) has found that the 
amount of moisture in the soil at the time of wilting is most in 
the case of hydrophytes and decreases through xerophytes. Living- 
ston (7), comparing the zerophytes - Allonia, Bouhavia and Eu- 
phorbia with the mesophytes - Phaseolus, Helianthus and Vicia, - 
that the desert forms show an adaptation to live in drier soils 
than plants of a humid climate. 

It has been found by Schroéder that the water content 
of the plant is the same during the growing time regardless of 
the conditions under which it was grown or the water content of 
the soil. It has been thought that a relationship existed be- 
tween the water content of the soil and that of the leaves. In 
plants the water content may vary within considerable limits 
without affecting in any marked way its life processes. The 
water content of the soil so long as it affects the water content 


of the plant within the limits of this critical range are without 


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material effect on the vegetal covering. A lowering of the 
water content of the soil that will affect the water content of 
the plant within the limits of its critical range, at once brings 


about changes in growth and finally wilting in the plant. 
II. METHODS. 


The plants were grown in pure quartz sand obtained 
from Ottawa, Ill. It had a water holding capacity of 81.- grams 
per 100 grams. The sand was used in glazed earthenware and glass 
containers. The earthenware jars were 5 inches in diameter, by 
3% inches deep; the glass Petri dishes were 7 inches in diameter 
and 1 inch deep. 

The containers were weighed and the weight recorded. 
They were then filled with the quartz sand and again weighed, 
and the weights recorded. This sand was then poured out into a 
pan and the amount of water added to give the desired per cent of 
soil moisture. This was mixed thoroughly so as to insure an even 
distribution of the water. The moist sand was now replaced in 
the container and beans, sunflower, or diseased and disease free 
wera; were planted. 

The containers were weighed at twelve hour intervals 
and the necessary water added to bring the weight back to normal. 


This was kept up until the plants were put under the fan for the 


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purpose of testing the wilting coefficient. At first the plants 
were grown in the green house but due to the dry hot air, those 
in the 10% and 20% moisture content wilted. This wilting would 
inhibit the normal growth of the plant by reducing for a time the 
normal turgor of the plant cells. To prevent this the plants 
were grown in a trough at ft. x 4 ft. x 8 inches, and covered 
with glass. The relative humidity in this chamber was between 
30% and 40%, as compared with the 20% and 25% in the laboratory. 
The earthen crocks used at first proved unsatisfactory 
because of the time necessary to bring the sand down to the wilt- 
ing coefficient in the tests. To avoid this time factor, flat 
granite pans 7 in. x 11 in. x 1 in. were used. After running 
several series in the pans it was found that the pans were too 
shallow, that is, they exposed too much surface for evaporation. 
Preliminary experiments clearly demonstrated that the plants 
grown in these pans showed a wide difference in the time and 
degree of wilting an consequently made it impossible to determine 
the residual moisture in the sand at the time of wilting of the 
different individual plants. In the experiments that followed 
single plants were grown in glass Soyka dishes 3 inches in dia- 
meter and 14 inches in depth. 
The moisture content of the sand used in the different 
series was 10%, 20%, 40%, and 60% of its water holding capacity. 


Beans, corn, and sunflowers planted in these dishes were allowed 


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to grow from a week to ten days before they were placed at a 
definite distance from an electric fan, and in the path of the 
air current. When the plants were placed before the fan, water 
was withheld. They were weighed every three hours and the rate 
of loss in moisture was determined. When permanent wilting took 
place, the plant was removed, and the sand was air-dried and 
weighed. 

In another series corn was grown under glass jars, 
at a relative humidity of about 80% or 90%, where the rate of 
transpiration was low. The plants grew maeaats and were from 
one to two days ahead of the corn grown otherwise. These seed- 
lings, when placed in the air current from the fan showed a very 
| marked difference in the wilting coefficient. In place of the 
Plants wilting and falling over as in the first series, they dried 
retaining the green color and not "firing" in any way. The plants 
were permanently wilted in from 5 to 7 hours after being placed 
| before the fan. 

In nearly all cases, the leaves of the diseased corn 
j when placed in the air current would begin to "fire" at the tips 
and this would gradually enlarge, and at the time of wilting ex- 
| tended half way down the leaf. The stalks of the diseased corn 
would wilt and bend over at the Sand, while in the disease free 


;corn they would wilt at the insertion of the first leaves. In 


the sunflower the youngest plants would be the first to wilt, 


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while in the bean the oldest plants would wilt first. In the 


sunflower there was considerable loss of water from the stems 


as shown by the gradual shriveling. In the bean seedlings the 
cotyledons showed a similar shrinkage due to shriveling. The 
leaves seemed to retain their turgidity until the cotyledons 
were dried up, and then they rapidly wilted. The bean never 
fell over after permanent wilting. 

| There was little difference between the root develop- 
ment of the disease free and diseased corn. The most marked 
difference was in sand of 40% saturation. Here the root develop- 
ment of the diseased corn was very weak. The length of the roots 
in the disease free corn was from 1$ to 2 times that of the 
diseased. In sand of 60% saturation molds would form around the 
shoot of the diseased corn and the mycelium would branch out 
over the sand and grow up the stalk. 

In the carrying out the experiments considerable 

difficulty was met with in keeping the water content of the sand 


uniform throughout the dish. This was practically impossible to 


do for the roots would have the bottom of the sand dry, while 
evaporation from the surface would keep this below the moisture 
content. The decrease I got in the ability of my corn to take 
moisture from the soil was due to the physiological effect of 


moisture upon the fungus which causes root rot of corn. 


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Lil; “eRESULTS: 
Table 1. 
Average results obtained on series including 46 beans, 


57 sunflowers, and 93 diseased and disease free (Root Rot) corn. 


Plant Soil Moisture 
10% 20% 40% 60% 
Disease free corn ae -909 273 .45 
Diseased corn .894 1.08 .999 -495 
10% 20% 30% 40% 
Bean at 1.46 - 6.37 Oe oy = 
Sunflower .462 5 ga | 1.48 8.26 
Table 2. 


Results on bean and sunflower seedlings. 


Number Dry weight Plant Original water Percent of non- 
(grams) content available water 

16 - 1052 5.8: 40% 16.6 
18 1083 " 20% 2.06 

6 1092 " 40% aa Oe 

3 1101 " 10% .37 

5 1007 " 20% 1.03 

4 1061 " 20% 1.03 
10 1007 " 20% .69 

59 1073 Bean 20% dee 
60 1048 " 20% ule 
68 1093 i" 30% 6.89 

58 1134 " 20% 2.06 
73 1070 n 40% 1LOvS 

56 1120 " 10% ey 

55 1073 " 10% see 

57 1065 " 10% SOF 
67 1079 " 30% 6.55 
70 1079 " 40% 10.69 
15 1126 f 30% 6.89 


i aa 1047 " 40% 8.6 


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Table 3. (continued). 


Number Dry weight Plant Original water Percent of non- 


(crams) content available water 
69 1028 Bean 30% 5.8 
61 1107 " 30% 5.8 
63 10390 " 30% 6.5 
66 1114 " 40% a0 
64 1067 : 40% 1030 
52 1116 n 20% Sel 
65 1041 n 40% 1Ox3 
62 1075 i 30% Soe 
53 1103 " 20% 1.03 


IV. CONCLUSIONS. 


Individual plants show a wide variation to drought 
resistance. This is due in part to the more efficient absorptive 
power of the roots of some which can overcome to a greater degree 
than others the physical forces that hold the water of the soil. 
In general it may be stated that under similar conditions differ- 
ent plants have different wilting coefficients in the same texture 
of soil. 

In making wilting coefficient determinations the follow- 
ing precautions should be observed: 

1. The soil used should be of uniform texture. 
a. The soil should be brought to a uniform water content before 


being used. 


5. The moisture lost by transpiration should be replaced at 


frequent intervals. 


| ae oe a =o =a —— | 
| 


SORES MOS, & @idez 


orn .to saeoiet tecsw fateps79 TriaeS “i 
seien Sligi ta : Taedaon . 


a 
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mo 
inagioae 
iy 
Wo 


ear are se ae 


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‘on sd? of Igege at exh es ade 


wend retest »t gmanievo neo Soci eyed to stoeg 


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vee ots2bnoo te thoke eemet caey besute ad © vw @ 


; 

i 

i ufxe* ose en? ol ecasfoLiteed alee jrevei2Ts 
. 

: 


. 
a 
. 

. 

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z 

. 

is 


t 


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” 


ay kr 
7 = a 
> % 
a ) anes 
Refs SUP eee F, 
ae: ie lea. 
——_— = _ = a” - 


All sudden fluctuations in temperature should be avoided. 
5. The moisture determination should be made as soon as the 
plant has reached the stage of permanent wilting. 

There is a marked difference in the root development 
of different plants. This difference may become more marked 
under certain soil moistures. Thus in a soil of 10%, 20%, 30%, 
and 40% moisture content the root systems of the bean and sun- 
flower seedlings showed essentially the same development, but 
the bean seedling had a lower wilting coefficient, than the sun- 
flower seedling at 10%, while at 20%, 30%, and 40% moisture con- 
tent the sunflower had the lower wilting coefficient. 

This shows conclusively that the wilting is dependent 
on the absorptive power of the roots rather than on root develop- 
ment. This was well brought out in the case where sunflower and 
bean seedlings were grown in the same pot under identical condi- 
tions. Where the plants were of uniform height and vigor the 
bean seedlings under certain conditions, would wilt from one to 
three hours before the sunflower. This may in part be due to the 
difference in structure. In the case of the bean the cotyledons 
would dry up before the leaves would show signs of wilting. 

One may conclude that the difference in the wilting 
coefficients of plants is due to a difference in the absorptive 
powers of the roots rather than to differences in development, 


which in a way may affect the wilting coefficient. 


a eed. Stipes olin it nl ce hn pa 


-_ —* ne ho a, 
- é ee ee ee 


> 


“~ 


ipa 


van an, oe ’ “- ; 


“75 


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s eLisdw \60L te ae. 


72 a ed ating to. 


ee 
5 taboo CSgm . 
LPs * ten wits 


wol su Bef ORG: 


ei? erote = 


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—: 


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a > a 
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= 14 


V. LITERATURE CITED. 


1. Sachs. Tobacco plants in different soils. 1859. 

8. Briggs, L. J. & Shantz, H. L. The wilting coefficient for 
different plants and its indirect determination. 
Ueno weow. of Agr. Bur. of Plant Industry. Bull. 230.:.1912 

&. Blackman, V. H. The wilting coefficient of soils. | 
dour. of Ecology: Vol. 42.0 19i4. 

4. Caldwell, J. S. The relation of environmental conditions to 
the phenomenon of permanent wilting in plants. 
Physiological Researches. No. 1. 1913. 

5. Crump, W. B. The coefficient of humidity. 
New Phytologist 12:125. 1913. 

6. Hedgcock, G. ©. The relation of the water content of the 
soil to certain plants. Bot. Survey of Nebr. Vol.6. 

7. Livingston, B. E. & Brown, W. H. Relation of the daily march 
of transpiration to variations in the water content of 


foliage leaves. Bot. Gaz. 52:309. 1912. 


UNIVERSITY OF ILLINOIS-URBANA 


ii | 


30112 


